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Factors Governing Paint 
Consistency 



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The \«u Jersey Zinc < Iompan> 

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160 Front Street, New York 

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1138 Guardian Buil 

i The New Jersey Zin< 



[Reprinted from Industrial and Engineering Chemistry, Vol. 13, Xo. 2, page 122. February, 1923.] 

Factors Governing Paint Consistency 1 

By HKXRY GREEN 

\rch Laboratory, New Jersey Zinc Co., Palmertox, Pa. 

In this paper we have described the four factors on which the consistency of paint depends, 
more particularly the factor — flocculation in the pigment-vehicle system. Following a dis- 
cussion of the force of flocculation. Us influence on yield value and /nobility, and the great 
change in paint consistency which con be brought about by a heavy bodied oil, such as poppy- 
seed ail, we hare tried to show whether or not it is possible for paints with the same yield 
value and mobility to have different coi The value of such a study to point 

technology has been pointed out. 

THE FOUR FACTORS GOVERNING PAINT CONSISTENCY 2 

It has been shown thai paint is not a truly viscous liquid, but instead 
a slightly plastic material; 3 further, it has been demonstrated that if its 
flow through a capillary tube could be maintained completely telescopic 
throughout the lowest pressures, then its pressure-rate-of-flow curve would 
be linear and intersect the pressure axis at the right of the origin 4 I Fig. 1). 
From these facts it follows that for a given paint there is a definite shearing 
stress below which telescopic flow cannot take place. This stress has been 
called the yield mine The slope of the curve, Fig. 1, is a. function of the 
mobility of the paint; it may be considered as the reciprocal of viscosity, 
though it is not strictly correct to apply this term to plastic materials. 5 

All resistances to paint flow, regardless of their inception or manner of 
functioning, will be recorded either in the yield value or mobility, or 
jointly in both. On the basis of this, a tentative definition of paint con- 
sistency will be made: The consistency of paint is that property imparted to 
it by virtue of its yield value and mobility. 

Before analyzing this definition, in order to ascertain if it is possible to 
have two paints of the same yield value and mobility and still recognize 
them as having different consistencies, it will be necessary to Mudy the 
governing factors on which yield value and mobility depend. These 
factors are (1) the viscosity of the vehicle, (2) the pigment-vehicle ratio, 
(3) the force of flocculation in the pigment-vehicle system, (4) particle 
size of the pigment. 

1 Received August 22, 1922. Presented before the Division of Industrial and 
Engineering Chemist! 64th Meeting of the American Chemical Society, Pitts- 
burgh, Pa., September 4 to 8, 1922 

2 The expression paint consistency "refers to the consistency of any paint, whether i1 
be good or poor; it is ao1 h> In- confused with "painting consistency," which is simply 
a special case in which the consistency is considered right for painting purposes. 

s Bingham and Green, Proc. Am. Soc. Testing Materials, 19 (1919, 640 

* Green, Ibid., 20 (1920), 451. 

6 Bingham has given the name ''rigidity" to the reciprocal of "mobilil 



That the consistency of paint depends on (1) and (2) is self-evident and 
has always been so regarded. The importance of (3), however has been 
overlooked; and. as (4) is closely connected with (3) 3 the real function of 

(4) has not been fully appreciated. 
In regard to (1), it will be shown that 
it is erroneous to suppose that in even- 
case the more viscous the vehicle used, 
the less mobile the paint produced. 
ining (2), it is obvious that the 
yield value will vary directly and the 
mobility inversely as some function 
of the percentage pigmenl content. 
Experimental evidence of this has 
been given elsewhere. 4 



Prcjj u re. 



Ytcfd Value. 

Fig. 1 — Pressure-rate-of-flow curve for paint, 
assuming flow to be telescopic throughout 
range 



PIGMENT FLOCCULATK »N 

As (3) is one of the most important of the four factors, it is n< i 
to consider it in detail. 



Definition of Flocculatiox 

In order that no misunderstanding may arise as to the meaning of the term "floccula- 
tion," it musi be realized that it is used in this paper in a particular sense, and under no 
circumstance will it be permissible to substitute an}* other picture or model of this 
condition of aggregation than the one given here. Admittedly, it is used in an arbitrary 
manner, for which an apology is unnecessary as there is no choice left in the matter 
to one who wishes to U- perfectly definite in regard to this important phenomenon. 

Flocculation is a condition of aggregation; it is that condition or state of affairs 
which ha< arisen when the dispersed p] ormly dispersed and i 

in grou] - or clusters, the individual units (particles) of which are closely held together 
apparently by the residual surface tension existing in the mi. . q the disp 

and continuous pi 

Y\ hen the word "flocculation" is used it implies three things: (1) a previou 
having existed in which the discontinuous phase was dispersed in a continuous phase; 
(2) the units of the discontinuous phase 

brought into com art with each other (owing _ 

to convection current s, mechanically produced 
motion, etc.) forming groups (flocculates); 
and (3) adherence (apparently due to surface 
tension' of the touching units. 

Fig. '1 shows an undispersed lump of pig- 
ment immersed in a liquid vehicle. The 
vehicle is in contact only with the exterior of 
the lump; the interior contains air spaces. 
By mechanical means this lump is broken up 
and completely dispersed (Fig. 3). In this 
illustration the air spaces have lieen elimi- 
nated and the vehicle is presumably in 
complete contact with the 
particle. Assuming th effect 

and there still remains :i sufficient amount 
of surface energy in the pigmenl vehicle 
interface, then, if these i at 
adhere (presumably owing to tl 
intervening _ ,1, ,(„. ve \ 




lump 



Furthermore, even between the apparent points cle contact there probably 

exists a film of vehiele. There is no penetration of the vehicle into the intermolecular 
spaces ot the particle. The particles do not get so closely together that they cohere, 
forming larger ones. Flocculates in no way behave as large individual particles. 

The author has shown that under low pressures paint How through a 
capillary is nol telescopic, and. hence, the actual curve produced in 
practice is such as that shown in Fig. 5. By studying under the microscope 
the flow of paint through capillaries, it was possible to see that when low 
pressures were applied, corresponding to Branch a, internal telescopic 
shearing did not take place and the material (owing to slippage) moved in a 
solid mass Fig. 6). This cessation of telescopic shearing was caused by 
the fact that the pigment was flocculated — a natural condition m paints - 
and that the force of flocculation was sufficient to hold the mass intact. 6 
When sufficiently high pressures were applied, this force could be over- 
come, the material sheared, and Branch /> of the curve produced 1 m 7 

The upper branch of the curve was pushed away from the origin on 
account of the introduction of the lower one. If a had not been produced 
in actual practice), h would have intersected the origin and the yield 
value would have l>een zero; hut as a was caused mostly, if not entirely — 
shown below — by flocculation, we are forced to the conclusion that yield 
value itself is due principally to the force of flocculation which holds the 
pigment particles together. 

If Sulman's theory 7 is accepted as the explanation of flocculation, this 
force resides in the solid-liquid interface, and is due to incompleteness of 
wetting. The less I he degree of wetting the greater the force of flocculation 
and consequently the greater the yield value. 

__ _ In order to fully appreciate the 

importance of flocculation to paint 
consistency it is necessary to perform 
the following exp< riment: 

Make up a semipaste of zinc oxide with 
either kerosene or Nujol (two poorly wetting 
oils). It will be noticed in the process of 
mixing vehicle and pigment, that an abnormal 
quantity ot the liquid can l»e added without 
producing a mixture with a tendency to 
flow out under its own weight. This paste 
will be decidedly plastic and possesses a 
yield value When the consistency is cor- 
rect for the experiment, the mass should be 
Fig, 3— Particles dispersed sufficiently plastic to retain its form, as 

shown in log. 8. Now add a drop or two of 
bodied (heat-thickened or air-blown) poppy-seed ml and rub in well with a spatula. 
The effect is striking; the paste loses its yield value and becomes extremely fluid 
(Fig. 9). It no longer bears the slightest resemblance to its former consistency; in tact, 

6 The reason for the gradual discontinuance of telescopic flow undei 
pressures is given l>\ I. Buckingham, Proc. Am. Soc. Testing Material, 'J I L921 L154 

7 "A Contribution to the Sudy of Flotation," Bull Inst. Mining Metal., lv_> ptpj 

3 








the entire range in consistency from one extreme to the other seems to have been passed 
through by tin- mere addition oi but a few drops of heavy-bodied oil. 8 

This phenomenon is not a case of the fluidity of the added poppy-seed 
oil being so great as to cause a thinning of the mixture, for, in the first 

place, only a relatively small amount 

- — is added, and, secondly, the fluidity 
— — of this oil is actually less than the mo- 

- — bility of the mixture of zinc oxide and 
— kerosen< i Nujol. In other words, if 

^_"~_ it were to produce :m\ ' all, 

on the basis of tin- reasoning, it 

should l»e one of thickening rather 
_ I than thinning of the paste. How, 

- - then, are we to arrive at a 5S 
3." factory explanation? With the knowl- 
edge thai flocculation is the principal 
cause of yield value in painl 

thai the degree of flocculation is a function of wetting of pigmenl with 
vehicle, it is nol difficult to suppose thai the poppy-seed oil has dim- 
inished the surface energy in the solid-liquid interface and thereby 
produced deflocculation. 

Tin flocculated pigment is tin "structun " which holds tin ma 
■ n it a plastic in/tun: when this structun is destroyed plasticity tend 
vanish. 

It is nol necessary to remain content with this statement as a pure 
theory, for it i- easily verified as a fact. Fig. 10 ie a photomicrograph, 
under low power, of the zinc oxide and kerosene in it- original 
that is, as a plastic material : Fig. 1 1 
same after the addition of the 
poppy-seed oil; the first is floccu- 
lated and the second, deflocculated. 
Experimental dam. taken with a Bing- 
ham and l met* r, on t he 
mixtures of various pigments and oils 
before and after deflocculation, ate 
given in Table I. Ii 

to lie, ff II, 

order pig- 

and vehi running 

of t 1 ,, |,j 

vain 

N 




PrtJJ i/ re. 



mixture where the yield value drops from o.77 to 0.054, the smaller quantity 
being scarcely measurable. In the case of the white leads with Nujol, 
and zinc oxide with linseed oil, wetting is fairly high to begin with; co 
quently, the effect of the poppy-seed oil is lessened. The residual yield 
value? in these cases ran be attributed either to imperfecl deflocculation or 
frictional 9 resistance between the particles from close parking. 

The importance of flocculation to the phenomenon of "oil absorption" 
should be apparent from a study of photographs I _- 3 ind 9). Both 
mixtures possess practically the same pigment-vehicle ratio, but the 
pigment in Fig. 8 could l>e made to absorb many time- it- present volume 
of oil before it would assume the fluid appearance shown in Fig I. \ The 
flocculated mas- acts Like a sponge. The specific surface of the pigment — 
usually granted to lie of major importance in oil absorption -is the same 
in both cases, and ther< fore cannot be regarded as the deciding factor. 






.vim. k » 






&iw& 



il^fit ^ 




1 1 t in capillai \ I ube 1 h< pigmenl 
particles are flocculated and the flocculates interlocked, thus holding the 
ntact undei 



EFFECT OF SIZE OF PARTICLE <>X CONSISTENCY OF PAINT 

It should now be possible to obtain a fuller understanding of the 
manner in which panicle size of pigment affects the consistency of paint. 

Let us consider two zinc oxide paints ground in the same vehicle and 
in the same proportions; further, let it be assumed that the degree of 
wetting is identical for both pigments. The only difference is to be one of 
particle size. Lei the first one be a coarse-grained zinc oxide, and the 
second, a fine one- Bear in mind that when true How takes place the 
flocculates are sheared : thai the particles are thereby pulled away from the 
ones to which they wi ilated; and that a resistance to "pulling 

away" is set up, because of the surface energy residing in the interface. 
It is then clear that the more extensive the pigmenl surface per unit area 

9 E. C. Bingham, Bur. Standards, Sci. Papa 278 



I 

Will 

will 
















particles flocculate, forming a structure, a -till greater resistance is de- 
veloped, and this must be met with a still greater applied force if the 
original (and required) velocity is to be maintained intact. Eence, floc- 
culation should cause an increase in viscosity, or more correctly speaking 
with regard to paints — a decrease in mobility. 

Table I — Effect of Deflocculatiov on Yield Value \\i> Mobility 

Yield Temperaf ure 

Value Mobility Density C 

Nujol, B. C. W. 1 1 sti 1 1<> 2 59 22 

+ 

Poppy-seed Oil.. 1 17 n I H 22 

Xujoi, S. \Y. L 117 0.188 2 00 21.7 

+ 

Poppy-seed Oil n L99 245 21 65 

Nujol, Lithopone 3 77 202 1 2:; 21 8 

+ 

Poppv-seed Oil 0.054 0.529 21.8 

Nujol, Zno :; 17 188 1 35 

+ 

Poppv-srrd Oil 'to i) 302 22 9 

* + 

Poppv-seed Oil 25 342 

Raw Linseed Oil, ZnO 1 56 103 1 83 21 9 

+ 

Poppv-seed Oil I) S.">7 (I If,;, Jl [\ 

KAPXX, Acid No. 6, ZnO 3 17 238 2 02 22 

+ 

Poppy-seed oil 2 :.7 261 22 

Lidity (Mobility) 0.0792 

Poppv-seed oil Viscosity 0.00126 

k Temperature, °C 21.9 

In order to secure experimental evidence that this line of reasoning has 
led to a correel conclusion, it is necessary tu work with a system in which 
it is possible to alter the state of flocculation without appreciably 
changing its composition. This is accomplished most successfully by 
working with mixtures of either lithopone or zinc oxide and Nujol, where 
the effects of deflocculation can be exaggerated. The method, of course, 
is by working backwards— i. e., starting with a flocculated mixture, 
measuring it- mobility, deflocculating with poppy-seed oil. measuring 
again and noting the difference, if any. Table I records the data of this 

Ies of experiment-. It will he observed that in each case defloeenlation 
causes an menus, in mobility; this is tantamount to a decrease of mobility 
on flocculation. 

\ \l I E TO PAINT TECHNOLOGY 

\\ . are now in a position to realize the value that the study of floccula- 
te i- to painl technology. It has Keen shown that flocculation materially 
influences both mobility and yield value, and that these in turn define 
paint consistency. This fact alone is of great importance, hut perhaps 

7 



even greater is the radical change thai can be produced in consistency by 
the application of literally homeopathic doses of a heavj'-bodied oil. 
Poppy-seed oil is by no means the only oil that is capable of introducing 
this change, for, in fact, n< arly any properly bodied vegetable oil possesses 
this property to some extent. For this reason we cannot always assume 
that, of two paints, the one containing the more viscous vehicle will 
necessarily be the less mobile, because very viscous oils are often good 
deflocculators, and it would be conceivable that such a paint might even 
be more mobile than one formed from a less viscous but strongly flocculating 
oil. 

It might be worth while mentioning, at this point, that the Thinning of 
paints, either shortly after application and before drying, or while still in 
the container, is probably due to an increase in wetting power of the 
vehicle arising from the absorption of oxygen, most likely from the surface 
of the pigment itself. Increased wetting causes deflocculation, which in 
turn destroys the yield value of the paint, enabling it to How easily under 
its own weight. 




I ig 9 
Fig v -Plastic mixture oi zinc oxide and kerosene. Particles 
flocculated 
ter the addil all amounl • | oil 

Particles deflocculated 

Fig. 12 shows the effect on the plastic How curve of simple defloccula- 
tion— a decrease in yield value plus an increase in mobility. Fig. 13 shows 
the result from oxidation. If oxidation is slight it should i 
tically the same effect as illustrated in Fig. 12, for oxidation increases 
wetting; on the other hand, if the absorption oi i ontinues until 

vehicle becomes extremely viscous, then the mobility of the paint will fall. 
Paradoxically, such a paint has thinned and thickened at th. sanu iim< . 
In the case of soap formation, we have simply the addition oi m u panicles, 
either colloidal or crystalloidal, and perhaps the removal of a certain 
percentage of the finest gm< q1 particles. The net result is likely 

to be an increase in the total number of particles, which prod i me 

effect as increasing the pigment- vehicle ratio i. i the yield 

value and a lowering in mobility. 



B 



The object of this paper has been to discuss the four cardinal factors 
controlling paint consistency. Naturally there are others, such as tem- 
ture, for instance, but this may be called a minor one, for no inves- 
tigator is liable to judge consistency at any temperature that varies radically 
from normal room temperature; hence, the range is small and the effect of 
secondary importance. Soap formation has a pronounced influence on 
paint consistency, but this, in most cases, can be considered as a sub- 
division of factor (2) or (4). 










vmm 



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Fig. 10 — Photomicrograph of the plastic mixture shown in 
Fig. 8. Particles flocculated into dense im;i--i<» 



PROBLEMS OF GELATIXIZATIOX AXD STRINGINESS 

It is now necessary to consider the question raised in the opening- 
paragraphs. Is it possible to have two paints possessing identical yield 
values and mobilities, and at the same time recognize them as having 
different consistencies? As pointed out, all resistances to flow are recorded 
in the yield value and mobility; but do these quantities express such 
anomalies sometimes occurring in paint, as gelatinization and stringiness? 
It is certainly proper and permissible to speak of gelatinous and stringy 
consistencies. The phenomenon of gelatinization may manifest itself in 
two ways — either the vehicle gelatinizes or the pigment particles become 
covered with soap, firmly binding them together into an elastic mass. 
In both cases structure is formed which would increase the yield value and 
decrease the mobility. An attempt to secure measurements with the 
plastometer would only result in the breaking of this structure, which 
was a matter of slow growth, and hence it is problematical whether or not 

9 



the readings would be true values. Therefore, it will be a difficult matter 
to decide, from experimental evidence; if gelatinization is defined by and 
incorporated in the figures given by yield value and mobility. It might 
be well to point out that flocculates themselves are always dispersed during 
their passage through the capillary tube, but it is not actually the existence 
of the flocculates that influences yield value and mobility, but the force 
of flocculation, and this remains intact as long as the wetting properties of 
pigment and vehicle are unaltered. 



" 



Fig. 11 — Photomicrograph showing completeness of defloecu- 
lation after addition of poppy-seed oil 



If it should become possible to simulate gelatinization by a proper 
combination of the four governing factors, then certain yield values and 
mobilities would result that would henceforth be indicative of this type of 
consistency. Furthermore, if these yield values and mobilities should 
happen to match those of a truly gelatinized paint, then the tent:, 
definition of paint consistency would be immune from attack as far as 
gelatinization is concerned. 

At first glance these suppositions seem far removed from the realm of 
possibility, but deeper reflection will instill caution against arriving a1 a 
hasty and doubtful conclusion; it 
will develop the realization of the 
existence of undiscovered and latent 
possibilities, and emphasize the ex- 
treme difficulty of visualizing the 
mechanism of a model possessing the 
properties of a gelatinous paint. V 

10 




> ield 

value and m< 




I 



•mi no one bas produced any evidence indicating that the idea of 
simulating gelatinization is untenable, and so the question remain- an 
open one from this aspect. 

In i- e stringiness of paint-, the author believes that this depends 

on the fulfilment of four basic conditions: (1) comparatively high visc< 

■ icle 2 pigment deflocculated, 

pigmenl particles relatively fine 

and (4) low surface tension of vehicle 

against air. Care must be taken to 

gnize an essential distinction 
that the fulfilmenl conditions 

will not necessarily produce stringi- 

bu1 if stringiness is to be produced, then these four conditions 
must be maintained. For instance, an ordinarily high pigment-vehicle 
ratio could destroy stringiness, even though the four condition- were 
entirely satisfied. 

It will be observed thai the first three requirements directly g< 
yield value and mobility, and, if it were not tor the fourth, stringiness 
might be indicated in the measurements of painl consistency. If. however, 
a fundamental relationship exists between surface ten-ion of vehicle 

inst air and vehicle againsl pigment, then the fourth condition would 
be a function of flocculation and could not be raised as an objection to the 
definition. Fortunately, whether this relationship i 
range of variation in surface tension of the highly viscous paint vehicles 
i- narrow, and it would seem, from ;i practical viewpoint, to (><• impossible 
to produce two paints of equal yield values and mobilities, one of which is 

aid the Other not. 

In view of these considerations, the author fed.- that the definition of 
painl consistency may be accepted without qualification, at least until 
thee msiderably more experimental data on gelatinization and 

- available for study than at present. 



-May 15, 



11 



* 3.